Field of the Invention
The present invention relates to a process for producing 4,4'-dibromodiphenyl ether and, more particularly, to the uncatalyzed "neat" bromination of diphenyl ether followed by a subsequent digestion with methanol to produce 4,4'-dibromodiphenyl ether in very high yield and purity.
Description of the Prior Art
A number of historical approaches have been used in producing 4,4'-dibromodiphenyl ether. Thus, the product was first produced by brominating diphenyl ether in solution in carbon disulfide (Hoffmeister, Ann. 159, 210). Subsequent approaches involved bromination of diphenyl ether in carbon tetra-chloride or carbon disulfide in the presence of iodine (Mailhe, Murat, Comp.rend. 154, 603; B1.[4] 11, 332); diazotizing of 4'-bromo-4-amino diphenyl ether, conversion to diazonium perbromide, and subsequent decomposition of the latter with hot acetic acid (La Fevre, et al. Soc. 1927, 1171). A later approach involved treatment of diphenyl ether with bromine in glacial acetic acid in the presence of CuCO.sub.3 (Akchurin, C. A., 1946, 557); by heating the sodium salt of 4-(4-bromophenoxy)-benzene sulfonic acid with bromine in water at 50.degree. C. (Suter, Am. Soc. 53 (1931) 1112, 1114); and by heating of 4-(4-bromophenoxy)-phenyl phosphonic acid with bromine in water at 160.degree. C. (Davies, et al. Soc. 1932, 2880, 2881).
It has been suggested that bromination of diphenyl ether without catalysts produces a mixture containing 56-59 percent unreacted diphenyl ether, 36-40 percent monobromodiphenyl ether, and 3-6 mole percent of dibromo derivatives of diphenyl ether. The mono-bromo fraction contained 95-98 percent para-bromodiphenyl ether and 2-5 percent orthobromodiphenyl ether (Bakhvalov, O. V., et al., Zh. Org. Khim 1969, 5(2), 331-336; C.A. 70: 106118e);
German Offen. No. 1,930,594 describes the preparation of 4,4'-dibromodiphenyl ether in 90 percent yield by brominating diphenyl ether in liquid sulfur dioxide as a reaction medium at a temperature below 0.degree. C.
Still other approaches to the preparation of 4,4'-dibromodiphenyl ether involve condensation reactions. Thus, when bromobenzenes were heated with iodyl sulfate and then with concentrated, hydrochloric acid, R.sub.2 I.sup.+ Cl.sup.- was formed which, on coupling with para-bromophenol in aqueous solution containing sodium hydroxide, gave 4,4'-dibromodiphenyl ether. (Nilsson, C. A., et al., Chemosphere 1977, 6(9), 599-607; C.A. 88: 22273k). Another low yield synthesis of 4,4'-dibromodiphenyl ether utilizes coupling of para-dibromobenzene with phenol. Thus, heating para-dibromobenzene with phenol and potassium hydroxide in the presence of freshly precipitated copper at 180.degree. C. with continuous water removal gave 27 percent yield of para-dibromo diphenyl ether, with somewhat higher yields being obtained at higher temperature. (Bakhvalov, et al., Izv. Akad Nauk SSSR, Ser.Khim. 1970, (1), 143-5;, C.A. 72: 110930x). German Offen No. 2,242,519 describes the synthesis of 4,4'-dibromodiphenyl ether by a soventless Ullmann reaction of halobenzenes with alkali metal phenolates in the presence of cuprous or cupric oxide at 130.degree.-65.degree. C.
None of the foregoing preparations for 4,4'-dibromodiphenyl ether permit the desired production of 4,4'-dibromodiphenyl ether in very high isomer purity and yield.
Bromination of biphenyl with bromine vapor carried out in a vacuum dessicator gives 4,4'-dibromobiphenyl in 75-77 percent yield after recrystallization (Org.Syntheses, Coll. Vol. IV, pp. 256-258). However, the bromination of diphenyl is not readily comparable to bromination of diphenyl ether because of inconsistent and unpredictable formation of position isomers at various bromination levels.
"Neat" bromination (i.e., bromination of a substrate using stoichiometric amounts of bromine without solvent or other reaction medium) utilizing bromination catalysts has been described for a variety of higher brominated diphenyl ethers and biphenyls. For example, U.S. Pat. No. 4,214,103 describes the catalyzed neat bromination of diphenyl ether to produce partially brominated diphenyl ethers, especially brominated diphenyl ethers of five to eight bromine atoms per molecule, containing undesirable amounts of occluded free bromine, by-product hydrogen bromide, catalyst residue and other materials which adversely affect the color of the product and which may diminish its thermal stability. British Patent No. 1,436,657 relates to a liquid phase bromination of aromatic compounds (e.g., diphenyl ether) containing two or more non-condensed benzene nuclei in the presence of a halogen carrier (e.g., iron powder or the like) using an approximately 10 percent stoichiometric excess of bromine in the substantial absence of a diluent or solvent liquid to produce pentabromodiphenyl oxide.
British Patent No. 1,029,874 describes the production of tribromobiphenyl from the bromination of biphenyl with a stoichiometric amount of bromine in the presence of a bromination catalyst.
A number of other patents describe the catalyzed bromination of diphenyl oxide with or without a solvent to achieve bromination levels of 3, 4 and 6 bromine atoms or more (e.g., U.S. Pat. Nos. 3,285,965; 2,022,634: and British No. 1,472,383).
"Neat" bromination technology has been used for other materials as well in U.S. Pat. No. 3,192,272 (tribromination of dialkyl benzenes) and U.S. Pat. No. 3,366,694 (3-chloro-diphenyl ether). Various other patents describe the catalyzed perbromination of diphenyl ether under varying reaction conditions in the absence of a solvent or other diluent (e.g., U.S. Pat. Nos. 3,752,856; and 3,965,197; and British Patent Spec No. 1,411,524).
None of the foregoing prior art teaches the utilization of neat bromination technology to produce high yields of isomer specific 4,4'-dibromodiphenyl ether without catalysts, solvents and other reaction diluents.
Accordingly, the primary object of the present invention is to provide a new process for producing 4,4'-dibromodiphenyl ether.
A related object is the provision of a unit process for producing higher purity 4,4'-dibromodiphenyl ether in high yield and assay.
A still further object is the provision of a process of the character described which avoids the disadvantages of the various prior art techniques for producing 4,4'-dibromodiphenyl ether.